Fiber Bragg gratings (hereinafter referred to also as “fiber gratings” or simply “gratings”) are well known and widely used in a variety of optical applications. In general, a fiber grating is formed by providing a periodic variation in the refractive index of the core of an optical fiber. The periodic variations or gratings in the fiber core cause reflection of a particular Bragg wavelength given by λB=2nΛ, where n is the mean refractive index of the grating, and Λ is the grating period. All other incident wavelengths are transmitted through the grating.
Thus, it is well known that by choice of n and Λ, a fiber grating may be effectively utilized as an optical filter for filtering a desired wavelength from an optical signal. In fact, due to their narrow passband and relatively inexpensive cost to produce, fiber gratings have developed as key components of many fiber optic communication systems where wavelength or channel selection is critical. Fiber gratings are widely used, for example, for channel selection in wavelength division multiplexed (WDM) or dense wavelength division multiplexed (WDM) communication systems, wherein a plurality of distinct optical wavelengths or channels are multiplexed and propagated over an optical medium to a plurality of receivers. In these systems, the channels or wavelengths chosen for transmission, as well as the channel spacings, are selected to correspond to an International Telecommunication Union (ITU) channel grid, wherein channel spacing may be, for example, 50 or 100 GHz. Reliable selection of a particular ITU channel from a WDM signal is essential to proper functionality of a WDM system.
One difficulty associated with the use of fiber gratings for channel selection relates to the temperature dependence of Λ and n. Variations of Λ or n result in corresponding variations in the Bragg wavelength. In a typical fiber grating formed from germania doped fused silica fiber, the temperature dependance of the Bragg wavelength is dominated by the temperature variation of n, and may be approximately 0.0115 nm/° C. or approximately 1.44 GHz/° C. at a wavelength of 1550 nm. This translates to a wavelength variation of approximately 100 GHz over a 70° C. operating temperature range. In a WDM or DWDM communication system with a channel separation of 50 or even 100 GHz, this temperature variation is clearly problematic.
Several fiber grating temperature compensation schemes have been proposed and attempted. One approach has been to provide an external heat source which includes electronics for providing thermal stabilization of the grating. Such heat sources and associated temperature-control circuitry, however, add significant costs, increase system complexity and consume power.
Accordingly, there have been several attempts to provide a temperature-dependant compensating strain to the fiber grating. Gratings of this type are generally referred to as temperature-compensated gratings, and generally do not require thermal stabilization, i.e., from an external heat source. Some common temperature-compensated grating schemes utilize re-entrant tubes or other structures made from materials with dissimilar thermal expansion characteristics, bimetallic strips, or negative thermal-expansion ceramics. To date, however, each of the known approaches to providing a temperature-compensated grating have failed to provide a sufficiently reliable and cost-effective device.
Accordingly, there is a need in the art for a temperature-compensated grating package which provides reliable compensation for the temperature dependency of the Bragg wavelength. There is a further need in the art for a temperature-compensated grating package which may be efficiently and cost-effectively produced.